1. Field of the Invention
This invention relates to a system and method for molding parts, and more particularly, a system and method for molding parts from contaminated molding materials using a single thermal heat rise.
2. Description of Related Art
In the field of thermoplastic molding, it is common to mold parts using either an injection or compression molding process. Due to the size of the orifices used in the injection molding equipment, it is often difficult to injection mold with reinforcing fibers, such as glass fibers, having a length over one-eighth inch because such fibers are not easily injected into or conveyed through the injection mold equipment. In addition, it is difficult to use contaminated molding materials such as those collected in plastics recycling programs unless they have been substantially cleaned, processed and put into a usable form and size prior to being used in the injection molding equipment. Such cleaning and processing are expensive and can substantially increase the cost of using the contaminated materials, thereby making them economically impractical.
Another problem with the thermoplastic processes of the past is that the thermoplastics become degraded and lose, for example, their strength when exposed to multiple heat rises.
In general, there are two basic types of compression molding processes which may be used for molding thermoplastics. First, a sheet molding process involves placing a reinforcement, such as a glass mat, between sandwiching layers of a thermoplastic and heating the materials to produce a single sheet of material. The single sheet of material is then cut to the desired size and then reheated to molding temperature before being placed in a compression molding press. This process has the disadvantage of higher cost because of the apparatus required, the material handling costs incurred in making the sheet, handling and cutting the sheet, and the like. The material used to make the sheet is also subject to three thermodynamic cycles, a first cycle when the thermoplastic sheet is formed, a second cycle when the thermoplastic sheets and glass mat are molded together, and a third cycle when the resulting sheet is heated to molten temperature prior to molding the part.
The second form of thermoplastic compression is bulk molding compounds by producing a billet of molten material that is placed into a compression molding press which molds the molten material into a part. Effectively placing and distributing long reinforcing fibers in the billet has heretofore required complex machinery. For example, U.S. Pat. No. 5,165,941 issued to Ronald C. Hawley on Nov. 24, 1992, discloses an extruder apparatus and process for compounding thermoplastic resin and fibers. The Hawley extruder includes an apparatus for compounding thermoplastic resin and reinforcing fibers incorporating a resin extruder in which thermoplastic resin pellets are melted in a second, compounding, extruder in which the molten thermoplastic resin is mixed in intimate contact with long reinforcing fibers. The melted thermoplastic resin is not fed into the device with the fibers, but rather is introduced into the compounding extruder at a point downstream of the inlet point for reinforcing fibers, so that the fibers are mechanically worked and heated before coming into contact with heated, molten thermoplastic resins.
The Hawley device generally suffers from complexity that raises the investment and maintenance costs.
The compression molding of products using polymeric material and glass fibers has traditionally produced a material referred to as fiber glass reinforced plastic. This material exhibits characteristics better than the unreinforced plastics, but does not exhibit strength, elasticity or impact resistance comparable to thermoplastic materials which are specifically designed to exhibit these characteristics. Most fiberglass reinforced plastic currently in the market is thermoset and is essentially a solidified mixture of fiber glass and plastic without benefit of chemical bonding or specific methods of enhancing polymer entrapment of the glass fibers because the glass fibers are merely immobilized in the resin in which it is embodied. In addition, thermoset materials are generally not recyclable other than as filler materials, while thermoplastic materials can be remelted and remolded.
What is needed, therefore, is an apparatus and method for molding thermoplastic parts which is simple and economical and which preserves the length of the reinforcing fibers, evenly distributes the reinforcing fibers or any other filler materials while maintaining flexibility of the material type in products fabricated, is capable of compatibilizing various contaminated thermoplastics to allow use of post consumer recycled material, and which provides a compounding and fabrication environment which promotes chemical bonding and molecular orientation to enhance the characteristics of the molded part.